in the extreme North. 99 



Throughout all these geological ages, even in the extreme 

 north, the same types, distinctly marked, recur. The inter- 

 mediate forms that the variability of species would necessitate 

 are not met with there. 



In the second place, a whole series of new facts, established 

 by the recent discoveries, confirm the opinion that the glacial 

 zone must formerly have enjoyed a climate much warmer than 

 that which it has in our days. This fact springs from the 

 study of all the geological formations from the Carboniferous 

 epoch to the Miocene period. As the flora of the Carboniferous 

 is very different from that of our day, the inductions that we 

 derive from it are not, perhaps, very certain ; but the fact that 

 it consisted in great part of trees enables us to conclude with 

 certainty that the temperature was higher than at present. 

 The present limit of trees nearly coincides in the north with 

 the isothermal line of 10° C. ( = 50° F.) for July and August; 

 that is to say, those two months must have a mean temperature 

 of at least 10° C. in order that trees may live. Further north 

 life is impossible to trees. In the northern hemisphere the 

 normal limit of trees nearly follows the polar circle. But upon 

 this limit we find nothing more than a few scattered conifers, 

 the birch, and the poplar, and even those plants are only 

 represented by stunted individuals. Upon Bear Island, 8° 

 further north, we find, on the contrary, in the Carboniferous 

 deposits, a whole series of acotyledonous trees which, at pre- 

 sent, grow for the most part in the tropics without any spe- 

 cies reaching the temperate climates of the northern hemi- 

 sphere. 



It would be rash to indicate a precise number for the tem- 

 perature of this epoch ; but we may assert boldly that the 

 Carboniferous flora of Bear Island does not in any way indi- 

 cate a temperature different from that ]3resupposed by the 

 Carboniferous flora of central Europe. The species are there 

 associated in the same manner ; the trunks there are of the 

 same thickness, and denote an equally luxuriant growth ; 

 nevertheless Bear Island is 28° further north than the Vosges, 

 where we find the same flora in the Lower Carboniferous. It 

 is therefore probable that at that epoch the earth was not yet 

 divided into zones as regards the distribution of heat. 



Other conditions, again, are presented by the Miocene period. 

 The climate of the polar zone must certainly have been warmer 

 at that epoch than at the present day ; but if we compare the 

 vegetation of those countries with that of Switzerland at the 

 same epoch, we shall be convinced that there the temperature 

 already diminished in advancing towards the north. The 

 palms in Germany attained a latitude only of 51^° N. ; the 



